I remember a time when Rehau vetted a wanna be Rehau fabricator and Rehau chose whether they supplied the fabricator as opposed to the fabricator choosing Rehau but that was 30 years ago so don't know if that's still the case, I would guess not reading your comment.
Rehau is a good profile but it's not IMO a stand out one from all the others like it was 30 years ago, all profiles bar duraflex are pretty much on a par now but as we in the trade always say an poor window fitted well is better than a good window fitted poorly and you can stretch that to how it's fabricated too, yes it's conceivable that you can get a decent profile but then poorly fabricated into a frame
Unfortunately it's probably going to be the same for every pvc and aluminium extruder, they sell a quality product but rely on fabricators to make a decent frame to uphold the name of the extruder like I mentioned above about Rehau.
From memory didn't Veka do something similar with 'Veka Network' approved fabricators and installers?
All I can advise is get onto the fabricator and get them to come and look at the issues, if no joy then maybe speak to Rehau, they could possibly send a technical rep out to take a look, I know Eurocell are very open to that to protect their name
